I make a retained mode-ish gui library. (I am not sure if it falls exactly into the tradition of retained mode gui, but it involves constructing trees of widge

I make a retained mode-ish gui library. (I am not sure if it falls exactly into the tradition of retained mode gui, but it involves constructing trees of widget objects, so—close enough.) Its design is fundamentally much simpler than that of the immediate mode UI libraries. There is no need for implicit state. There is no need for layout to be a core feature of the library; layout can be 'just another widget'. This opens up the possibility for different types of layouts: constraint-based, grid-based, etc. It's even possible to implement an imgui on top of this fairly easily; but not the other way around, because imgui has implicit global state to manage. Imgui is not slow, especially with GPUs. But it definitely loses on the simplicity front.